From the Introduction: "Congress and the Donald J. Trump Administration are debating enhancing and expanding barriers on the southwest border. The extent of these barriers, and how construction of these barriers will be funded has become a central part of the interactions between Congress and the Trump Administration on border security and funding legislation for the broader federal government. An authoritative compilation of data on the details of federal investment in border barriers is missing from the debate. [...] This report briefly contextualizes the history of U.S. enforcement of the U.S.-Mexico border, before turning to funding for border barriers within the contemporary period, accounting for changing appropriations structures."
CRS Report for Congress, R45888
Congressional Research Service: https://crsreports.congress.gov/